1. Field of the Invention
The present invention relates to a switching device, and more particularly to a switching device for controlling the supply of electric power from a power supply to a load by detecting an abnormality.
The present application is based on Japanese Patent Applications Nos. Hei. 11-50118 and 2000-16686, which are incorporated herein by reference.
2. Description of the Related Art
A switching device is used for controlling the supply of electric power from a power supply to each load in, for example, an automobile. A related switching device is comprised of a semiconductor element for effecting changeover between the supply and cutoff of electric power as well as its drive circuit; an abnormality determining circuit including a shunt resistor inserted between the power supply and the semiconductor element and a comparator for comparing the voltage across the shunt resistor; an A/D converter for effecting analog-digital conversion of a determination signal from the abnormality determining circuit; and a controlling device such as a microcomputer for controlling the on/off state of the semiconductor element by commanding the drive circuit on the basis of the converted determination signal.
As the drive circuit outputs a drive signal in response to a command signal from the controlling device, the semiconductor element is turned on, which in turn closes a power supply circuit from the power supply to the load, thereby supplying electric power to the load. At this time, if an abnormality such as an overcurrent occurs, on the basis of a voltage drop at the shunt resistor the abnormality determination circuit detects that an abnormal current exceeding a normal value has occurred, whereupon the controlling device stops the outputting of the drive signal to turn off the semiconductor element, i.e., sets it in a nonconducting state, thereby protecting the semiconductor element or the load.
With the related switching device, since the controlling device such as the microcomputer, the abnormality determination circuit including the shunt resistor, the A/D converter, and the like are required, there is a problem in that the device becomes complex.
It is an object of the present invention to provide a switching device which is simplified.
To this end, in accordance with a first aspect of the present invention, there is provided a switching device which comprises a driving device which outputs a drive signal in response to a drive command signal, a first switching device inserted in a power supply circuit connecting a power supply and a load, and adapted to conduct in response to the drive signal and close the power supply circuit, a reference resistor which generates a reference voltage by receiving a current from the power supply, a second switching device inserted in a shunt circuit connecting the power supply and the reference resistor, and adapted to conduct in response to the drive signal and close the shunt circuit, an abnormality determining device which determines an abnormality by comparing an output voltage from the first switching device and the reference voltage so as to stop the driving device from outputting the drive signal, an on/off counter circuit which outputs a latch command signal when the first switching device has repeated conduction and nonconduction a set number of times in accordance with the state of the load by monitoring the state of the first switching device, and a cutoff latch circuit which renders the first switching device in a nonconducting state in response to the latch command signal, and latches the nonconducting state.
In the above-described switching device, when a current larger than during normal times flows across a power supply circuit during the occurrence of an abnormality such as a shortcircuiting of the power supply circuit, since the output voltage at the first switching device becomes lower than the reference voltage, the abnormality determining device outputs a determination signal representative of an abnormality to the drive circuit. The drive circuit stops outputting the drive signal, and the first switching device is turned off, i.e., is set in the nonconducting state, thereby stopping the supply of electric power from the power supply to the load. When the supply of electric power is stopped, and the output voltage at the first switching device and the output voltage at the second switching device drop to substantially the same level, the abnormality determining device outputs a determination signal representative of the normal state, and as the driving device outputs the drive signal again, the first switching device is turned on, thereby supplying electric power to the load. As the outputting and stopping of the drive signal are repeated, the first switching device repeats conduction and nonconduction, the frequency of this repetition is counted by the on/off counter circuit. When a predetermined frequency is reached, the first switching device is latched in the off state by the cutoff latch circuit.
Namely, control of the supply of electric power to the load and the cutoff of the supply of electric power during the occurrence of an abnormality is controlled by hardware circuits alone without using a controlling device such as a microcomputer, an abnormality determination circuit including a shunt resistor, an A/D converter, etc.. Further, most of the component elements such as the hardware circuits making up the switching device can be integrated in a single semiconductor chip. In short, the switching device can be simplified.
With the switching device in accordance with the above-described first aspect of the present invention, the determination signal from the abnormality determining device and information on the execution of the latch by the cutoff latch device are arranged to be outputted outside the semiconductor chip, for instance, to such as a microcomputer for managing the switching of each load, and the semiconductor is provided with a plurality of terminals including output terminals therefor.
Accordingly, in accordance with a second aspect of the present invention, there is provided a switching device which comprises a commanding device which commands the driving of a load, a driving device which outputs a drive signal in response to a drive command signal from the commanding device, a first switching device inserted in a power supply circuit connecting a power supply and the load, and adapted to conduct in response to the drive signal and close the power supply circuit, a reference resistor which generates a reference voltage by receiving a current from the power supply, a second switching device inserted in a shunt circuit connecting the power supply and the reference resistor, and adapted to conduct in response to the drive signal and close the shunt circuit, an abnormality determining device which determines an abnormality by comparing an output voltage from the first switching device and the reference voltage, a cutoff latch device which changes the drive command signal outputted to the driving device into a stop command signal in response to an abnormality determination signal of the abnormality determining device, and latches the stop command signal, and a monitoring device which monitors a state of the load on the basis of a state of the command signal inputted to the driving device.
If the switching device is configured as described above, if an abnormality occurs in the load, the cutoff latch circuit changes the drive command signal to the stop command signal in response to the signal from the abnormality determining device, and latches the driving device in a state in which the output of the drive signal is stopped. Further, whether the command signal inputted to the driving device is the drive command signal or the stop command signal is monitored by the monitoring device. The monitoring device is able to detect the occurrence of an abnormality as the command signal inputted to the driving device is changed to the stop command signal when the commanding device is commanding the drive, and is also able to detect that the first switching device is being latched in the nonconducting state. For this reason, the outputting of information on abnormality determination and the execution of the latch can be effected by the terminal for imparting the command signal to the driving device. Hence, terminals for outputting the determination signal from the abnormality determining device and for outputting information on the execution of the latch by the cutoff latch device become unnecessary. In other words, it is possible to reduce the number of terminals of the semiconductor chip which increase connections and make the device complex. Namely, the switching device can be made further simplified.
In addition, in the above-described second aspect of the present invention, the abnormality determining device may include a first abnormality determining device which determines an abnormality when the output voltage from the first switching device is lower than the reference voltage.
Furthermore, in the above-described second aspect of the present invention, the abnormality determining device may include a second abnormality determining device which determines an abnormality when the output voltage from the first switching device is higher than the reference voltage.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the present invention when read in conjunction with the accompanying drawings.